Our studies concern analysis of the cyclic AMP-dependent phosphorylation system in the plasma membranes of nontransformed and transformed cells. We have found that there are striking differences in the phosphorylation of the membranes of 3T3 and SV 3T3/MCA 3T3 cell lines. Nontransformed cells phosphorylate a 24,000 molecular weight endogenous plasma membrane substrate; transformed cells do not. This defect in transformed cells may result from a lesion in the endogenous cyclic AMP-dependent protein kinase rather than a defect in the protein substrate. We are currently studying this enzyme in synchronized cell populations in an attempt to determine its role in growth control. Other studies are directed at establishing the topography of the protein kinase and its substrate in the plasma membrane.